Communication systems in which multiple communication devices mutually communicate through serial communication using photocouplers are known. For example, Patent Literature 1 discloses an air conditioning apparatus of a so-called separate type in which multiple indoor units are connected in parallel to an outdoor unit via a pair of commercial power lines (a power line and a common line) and a signal line. In this air conditioning apparatus, the outdoor unit and the multiple indoor units mutually communicate through serial communication using a current loop including the signal line and the common line. Here, each indoor unit is provided with a resistor of a high resistance (approximately several kΩ to several tens kΩ) on the current loop for protecting the circuit element of each indoor unit in the event that the power supply is improperly connected to the signal line.